The mind's eye can learn to see, even in those blind since birth

The blind have long used other senses -- including touch -- to compensate… (Anacleto Rapping )

People blind from birth can be taught to "see" images that are conveyed as sounds, says a new study that calls into question a longstanding belief about the limits of the human brain.

Devices that scan visual images and reinterpret regularities as sounds were used to retrain the brains of congenitally blind people in a study published this week in the journal Neuron. The authors -- at the Safra Center for Brain Science at Hebrew University in Israel -- put people who had been blind since birth through 70 hours of training with a visual-to-auditory sensory substitution device.

Initially, the subjects were able to distinguish among faces, houses, everyday objects, body shapes and textures. Eventually, they were able to read letters and words, identify facial expressions and locate people's positions. In one video, a blind person is shown a picture of a woman with a ponytail and identified the hairstyle.

Blind people have long used the capability to use another sensory perception to compensate for blindness: Braille and blind walking canes allow people without sight to read and navigate. But when the authors of the current study put subjects in a brain scanner, they gained insight into the process by which training with a sensory substitution device allowed the mind's eye to "see."

The human brain is a remarkably efficient and adaptable organ: when an appendage such as a hand is amputated, or a sensory perception such as sight is lost, the specialized regions of the brain in which input from the hand or the eyes is processed are reassigned to other duties.

But scientists have long believed that the brain's adaptability is limited by early conditions: when a person is born blind, the capacity of the brain's visual cortex to process sight never develops, scientists have believed. With that lost opportunity, a window is closed, and even if eyesight were to be restored, the visual cortex, they believed, would forever remain "blind" to images.

Not so, the current study finds. When blind subjects listened to the "soundscapes" that conveyed information about a visual image, they showed activation in their visual cortex. In fact, when sounds conveyed the shape of letters, subjects who had never "seen" a letter showed activation in a patch of the left ventral visual cortex that uniquely comes alive when people with normal vision read letters and words.

"The adult brain is more flexible than we thought," said co-author Amir Amedi. With the right training approaches and technologies, the brains of people who have been blind for a long time -- even since birth -- might be "reawakened" to the task of processing visual information, he said. For some, he added, such training might even restore some lost vision.

In fact, a 2008 study first offered tantalizing evidence that scientists were wrong in their long-held belief that a congenitally blind person could never have normal vision even if eyesight were restored. A young Indian woman identified only as SRD was born blind of dense congenital cataracts, but her vision was restored when she was 12 at the Iladevi Eye Clinic in Ahmedabad, India. Twenty years after she regained her eyesight, a team from the Massachusetts Institute of Technology discovered this rare instance of a congenitally blind person whose vision was restored, and tested her extensively to see how she saw.

SRD's sight was virtually normal, though the MIT team found that she did not use certain cognitive tricks that most sighted people use to make sense of conflicting visual cues.

Other interesting cases of visual impairment and the cognitive strategies that the blind and the sighted use to make sense of their world are discussed by neurologist Oliver Sacks in his book The Mind's Eye.